The present invention relates to a method and apparatus for acoustical logging, and more particularly, to a method and apparatus utilizing ultrasonic energy to detect the presence of vertical fractures and anomalies in the earth formations surrounding a borehole. It is of considerable importance in the petroleum industry to detect vertical fractures, since many formations produce hydrocarbons through fractured reservoirs. Most well logging devices are designed for responding to horizontally extending anomalies and therefore are not suitable for locating these vertical fractures.
Prior attempts at detecting the presence of vertical fractures generally have resorted to acoustic logging means. Typically, attempts have been made to produce acoustic waves that would travel circumferentially around the borehole to detectors where the acoustic waves would be received. Two types of acoustic waves are normally generated by the acoustic logging device, shear waves and compressional waves. Compressional waves will pass through a fluid filled fracture with little or no attenuation, while the shear waves, which are readily propagated only in solid materials, are significantly attenuated by the fluid filled fracture. Hence, in the prior art attempts, if a compressional wave, but no shear wave, was present, it was an indication that a vertical fracture was present in the formation. Prior attempts at detecting fractures in acoustic logging have failed in several respects. One approach, described in U.S. Pat. No. 2,934,694 sought to direct shear waves along the borehole wall. Variations of the received shear wave were observed as an indication of the presence of open fractures. The logging device utilized continuous acoustic waves, which gave rise to special transmission and separation problems. Physical separation of the transmitter and receiver was used to separate the shear wave from the compressional wave, it being recognized that the shear waves travel at a lesser velocity than does an associated compressional wave. Since different formation materials may significantly alter the speeds of propagation of the two types of waves, the approach was not entirely successful. U.S. Pat. No. 3,585,580 describes a device which sought to utilize the shear wave energy by projecting the acoustic wave into the formation at such an angle of incidence that the shear wave energy would be maximized while the compressional wave energy would be minimized. Rather than using a continuous acoustic signal, a narrow ultrasonic beam was used to generate the acoustic waves in the formation. Under the assumption that the compressional waves were essentially nonexistent, gating circuitry was used to initiate the recording of the received acoustic wave shortly after its arrival at a detector and to continue recording the wave for a period of predetermined duration based on the average velocity of the shear wave through the transmission medium. While this approach made improvements over the prior art, it was not altogether successful, possibly due to the arrival of compressional waves notwithstanding the particular angle of incidence used, or possibly due to the recordation of waves other than the shear wave during the fixed period of recordation. Another approach, described in U.S. Pat. No. 3,794,976 utilized substantially omni-directional transmitters to produce acoustical waves which travel through the formation about the circumference of the borehole. This device was directed particularly toward the detection of vertical fractues, using a particular angle of orientation of the transmitters with respect to the receivers to minimize the compressional wave effect. Thus, it was assumed that all waves received were shear waves. This approach suffered from some disadvantages, possibly due to the fact that the transmitting transducers were located a considerable distance from the surface of the borehole wall. In addition, some compressional wave energy was possibly received by the receiving transducers, resulting in erroneous interpretations. Still another approach utilized two sets of transmitting transducers in an attempt to detect vertical fractures. In the device described by U.S. Pat. No. 3,775,739, one set of transmitting transducers was oriented to provide substantially compressional waves in the formation while another set of transmitting transducers was oriented to provide substantially shear waves in the formation. This approach was a considerable improvement over the single omnidirectional transducer in that the shear waves and compressional waves were separately produced and recorded. While this approach improved the results, it also included the additional complication of having two sets of transmitting transducers and two sets of separate receiving transducers in place of a single-transmitting and single-receiving device.
Accordingly, it is an object of the present invention to overcome the difficulties of conventional systems for detecting vertical fractures in formations.
Another object of the invention is to provide a method and apparatus for detecting vertical fractures from electrical signals produced solely by shear waves transmitted through the formations adjacent the borehole.
Another object of the invention is to provide improved electrical circuitry for separating the shear waves from any later arriving compressional waves.
Another object of the invention is to provide improved electrical circuitry for detecting and recording guided fluid waves transmitted circumferentially about the borehole wall.
Another object of the invention is to provide novel electrical circuitry by which only one transmitter may become operative during any one time period.
Still another object of the invention is to provide novel electrical circuitry whereby parts of each or all of the shear waves and compressional waves resulting from each acoustic wave produced may be recorded for interpretation.